This application is directed towards methods of removing dissolved oxygen from water and to prevent the dissolution of negative valence sulfur bearing minerals, such as pyrite, in the subsurface. In particular, the methods are useful in making use of underground aquifers for water storage and for the subsequent recovery of this stored water for private or public use. The inventive methods are also useful for aquifer recharge wherein water is injected to increase water levels in an aquifer so that the water may be available at other sites where users may desire to produce from the aquifer or where recharge is used to prevent land subsidence. The water is prepared for injection or recharge by the addition of a small amount of sulfide compound to remove oxygen in order to prevent the mobilization of undesirable trace metals, such as arsenic, into the water.
Currently there are numerous systems for underground natural water storage. These include those taught in publicly available disclosures.
U.S. Pat. No. 7,192,218 describes an underground porosity water storage reservoir that minimizes the impacts on surface uses of the reservoir site. There is no discussion related to the prevention of leaching of unacceptable trace metals.
U.S. Pat. No. 4,254,831 describes a method and apparatus for restoring or maintaining an underground aquifer that is plagued with decreased water flow due to an accumulation of undesirable flow impeding agents in the aquifer. A series of injection wells are disclosed. Again, there is no discussion related to the prevention of mobilizating or leaching of unacceptable trace metals.
U.S. Pat. No. 7,138,060 discloses a method of in situ treatment of contaminated groundwater which includes identifying a site contaminated with a pollutant susceptible to degradation by sulfate reducing microorganisms. An amount of sulfate needed to metabolize the contaminants is estimated and applied. The sulfate concentration in solution is 1,000 ppm or more. This is a much higher concentration than would be acceptable for drinkable water or needed for the control of dissolved oxygen. The sulfate ion does not react with the dissolved oxygen as would a sulfide compound described herein.
The above mentioned patents do not disclose methods that provide for the control of oxygen in the water injected into a subsurface aquifer or the prevention of the dissolution of minerals in the subsurface that contain negative valence sulfides, such as pyrite, which contains arsenic. Lowering or eliminating the amount of dissolved oxygen will avoid trace metals, such as arsenic, from being mobilized into the underground water where such trace metals naturally exist in the natural underground aquifer rock material, i.e. strata.
The preservation and management of water resources has become an important focus of the environmental movement. In a great portion the United States and many other areas of the world, water is abundant only during seasonal periods. During these wet periods, there are often excess amounts of water, which is lost because it cannot be economically stored for use later during dry periods or periods of drought.
In recent years, newer technology has been developed that allows water to be captured and stored in ways that are more economical than traditional storage methods. The new technology involves capturing excess water and pumping it underground into certain subsurface geologic formations for storage. The water can later be recovered for use during dry periods. The process is commonly called aquifer storage and recovery (ASR) and it is conducted using wells for injection and recovery.
ASR is a now proven technology for storing large volumes of fresh water. There are sites where more than a billion gallons of water are stored and recovered annually by this process. Some common users of this technology are municipal water utilities and industry.
Pumping water into the ground is also done to recharge aquifer systems that are experiencing depletion due to over pumping. Injection of fresh water into aquifers is done to replenish aquifer systems for both environmental and human benefits. Protection of the quality of underground water resources is also an issue of extreme concern, and therefore regulations have been developed to control underground injection so that underground water resources can be protected. In the United States, the agency that regulates underground injection of water is the United States Environmental Protection Agency (EPA).
The EPA has adopted a water quality standard for potable water with regard to arsenic of 10 parts per billion (ppb) and other trace metals. In certain areas, arsenic mobilization in the subsurface affects the process of injecting water into the ground for storage and recharge. The regulations require that water may not be injected into an underground source of drinking water (USDW) if the act of injection causes the USDW to exceed a primary drinking water standard. It has been found in many cases that the injection of waters for the purpose of storage or recharge causes arsenic (and potentially other metals) within the aquifer to exceed drinking water standards. This is a violation of the EPA rules and therefore, where trace metals such as arsenic are released above regulatory limits, the practice of injection for storage or recharge must cease.
In most cases, where injection of water has caused a violation of the drinking water standard for arsenic, it is due to the oxidizing components of the injected water reacting with the natural minerals in the geologic formation and leaching or dissolving arsenic from its native state as a solid. Arsenic is often a trace constituent that occurs in pyrite in the subsurface. Studies have shown that the mobilization of arsenic is due, in part, to reactions with pyrite by such oxidizing constituents as dissolved oxygen, nitrates, and disinfectants such as the hypochlorite ion, chlorine, ozone that are present in the injected water. The oxidants react with subsurface minerals, such as pyrite, which are native to the subsurface environment. For pyrite, arsenic is the trace metal most commonly released in this process.
The present invention involves a unique method to treat the water to remove dissolved oxygen and other oxidants and thereby prevent the dissolution of subsurface negative valence subsurface sulfur based minerals, such as pyrite.
The present invention relates to the unique chemical properties of the sulfide ion—bisulfide ion—hydrogen sulfide chemical species, in low concentrations, which are injected into a water stream to remove dissolved oxygen, chlorine, and nitrogen oxides in the water prior to entry into a natural aquifer. The injected chemical also inhibits the dissolution of pyrite and similarly negative valence sulfur bearing minerals in subsurface geologic formations.